If i have lots of images appearing tumblr blogs, the sort of tumblogs with very little text just reams and reams of images for people to browse through (example - http://whereisthecool.com/). If my image is embeded in their site like this :

Also, you need to consider that Google operates functionally like the lowest common browsing denominator (like a Lynx browser, or as one SEO put it 'a blind five year old'), so if you ran a site through the Lynx browser, those ALT attributes are replacements for anchor text.